Essential but partially redundant roles for POU4F1/Brn-3a and POU4F2/Brn-3b transcription factors in the developing heart

Abstract

Congenital heart defects contribute to embryonic or neonatal lethality but due to the complexity of cardiac development, the molecular changes associated with such defects are not fully understood. Here, we report that transcription factors (TFs) Brn-3a (POU4F1) and Brn-3b (POU4F2) are important for normal cardiac development. Brn-3a directly represses Brn-3b promoter in cardiomyocytes and consequently Brn-3a knockout (KO) mutant hearts express increased Brn-3b mRNA during mid-gestation, which is linked to hyperplastic growth associated with elevated cyclin D1, a known Brn-3b target gene. However, during late gestation, Brn-3b can cooperate with p53 to enhance transcription of pro-apoptotic genes e.g. Bax, thereby increasing apoptosis and contribute to morphological defects such as non-compaction, ventricular wall/septal thinning and increased crypts/fissures, which may cause lethality of Brn-3a KO mutants soon after birth. Despite this, early embryonic lethality in e9.5 double KO (Brn-3a−/− : Brn-3b−/−) mutants indicate essential functions with partial redundancy during early embryogenesis. High conservation between mammals and zebrafish (ZF) Brn-3b (87%) or Brn-3a (76%) facilitated use of ZF embryos to study potential roles in developing heart. Double morphant embryos targeted with morpholino oligonucleotides to both TFs develop significant cardiac defects (looping abnormalities and valve defects) suggesting essential roles for Brn-3a and Brn-3b in developing hearts.

The POU4F2/Brn-3b transcription factor is required for the hypertrophic response to angiotensin II in the heart

“…Similarly, AngII stimulates Brn-3b mRNA and protein in hypertrophic H9c2 cells expressing β-MHC and displaying characteristic sarcomeric remodelling. Based on these findings and data from previous studies 40 , we determined that the rat heart-derived H9c2 cell line provides a useful adjunct for studying the effects of Brn-3b in cardiomyocytes 40,42 .…”

“…However, Brn-3b promotes cell proliferation/growth in other cells by activating cell cycle genes, cyclin D1/CDK4 34–37 but can confer drug resistance and migratory potential to cancer cells by activating the small heat shock protein, HSP27 and/or repressing adhesion molecule plakoglobin (γ-catenin) 38,39 . Yet, if Brn-3b is co-expressed with the growth inhibitory p53 protein, then it cooperates with p53 to maximally increase transcription of pro-apoptotic genes such as Bax, thereby increasing apoptosis 40–42 . Therefore, to understand its mechanism of action, it is important to identify tissue-specific effects of Brn-3b.…”

“…Brn-3b is highly expressed in foetal hearts with expression detected in cardiomyocytes in heart sections and primary isolated cultures of cardiomyocytes 43 . Although expressed at lower levels in adult hearts 42 , Brn-3b is increased throughout the myocardium by 24 h after coronary artery ligation 40 . Around the infarct zone, Brn-3b co-expression with p53 causes increases Bax protein linked to cardiomyocyte apoptosis.…”

“…Around the infarct zone, Brn-3b co-expression with p53 causes increases Bax protein linked to cardiomyocyte apoptosis. Brn-3b is required for maximal Bax induction when co-expressed with p53 in injured cardiomyocytes because reducing Brn-3b in primary neonatal rat ventricular cardiomyocytes (NRVM) cultures, using short interfering RNA (shRNA) blocks Bax expression, even when p53 expression is intact 40,42 . However, the effects of Brn-3b in non-injured myocardium, lacking p53 is still unknown and must be investigated.…”

Linking metabolic dysfunction with cardiovascular diseases: Brn-3b/POU4F2 transcription factor in cardiometabolic tissues in health and disease

“…Brn-3b is a tissue-specific TF [18][19][20][21][22][23][24][25] belonging to the class IV POU (Pit-1; Oct-1; Unc-86) sub-family of homeodomain TFs, and is characterised by a highly conserved POU domain that forms a helix-turn-helix (HTH) DNA binding structure 26 . The POU domain consists of a 60aa homeodomain, POU-HD , with strong similarity to homeobox proteins and 74-82aa POU-specific domain (POU s ), which is unique to POU proteins (Fig.…”